The invention relates to low-dust, well dispersible granulates based on silicate fillers modified with organosilicon compounds, their manufacture and use.
It is known to treat oxidic surfaces with organosilicon compounds in order, by means of this pre-treatment, to improve the bond between oxidic filler and organic polymers of the most varied chemical composition and hence to improve the strengthening properties of the fillers in the polymers. For this purpose it is possible, for example, to dissolve the organosilicon compound in question in an organic solvent and then treat clays, for example, with this solution (U.S. Pat. No. 3,227,675 HUBER). From U.S. Pat. No. 3,567,680 it is known to modify kaolins suspended in water with mercapto- and aminosilanes. The organosilicon compounds in question are water-soluble in the quantities required for modification, however, so that in this case also the treatment of the filler takes place from a solution.
U.S. Pat. No. 4,151,154 relates to oxidic silicate fillers whose surface is subjected to a treatment with two types of organosilicon compounds. The oxidic particles are treated in such a way that they have a greater affinity for water and can also be more easily dispersed in aqueous systems.
The use of organosilicon compounds containing sulphur in vulcanizable rubber mixtures is known from U.S. Pat. No. 4,076,550. This compound can also be used in mixtures with silica, but these are not thermally pre-treated and have only limited storage stability. EP-PS 0126 871 (U.S. Pat. No. 4,514,231) describes a process in which the surface of silicate fillers is modified with the aid of an aqueous emulsion of water-insoluble organosilicon compounds.
U.S. Pat. No. 4,141,751 relates to a process which is carried out entirely without solvents, but it is not practicable for certain organosilicon compounds, as practical experience shows.
EP 0442 143 B1 (U.S. Pat. No. 5,116,986) describes a process in which the silane is homogenized with the filler without solvent and water in a two-stage production process initially at room temperature in a heavy-duty mixer and then brought to reaction at elevated temperature.
Products produced according to this process have been on sale for about three years under the name of Coupsil(r). As they occur in powder form they must then be transformed into the desired low-dust form in an additional granulation process. If this is carried out according to the prior art, a less satisfactory dispersibility can be expected.
A further patent which describes the modification of silicate fillers with organosilanes is EP 0177 674 B1 (U.S. Pat. No. 4,704,414). In this case the silica is suspended in water, mixed with an aqueous silane emulsion with the addition of emulsifiers and then dried at elevated temperature. In this case also, an additional granulation process must follow, in order to produce a low-dust product, and this is known to impair the dispersibility of the modified material.
Products which are obtained according to patents EP 0442 143 and EP 0177 674 are characterized in that including after reaction of the alkoxysilyl group(s) of the organosilane with the silanol groups on the surface of the silicate filler used, which takes place with the liberation of an alcohol, a part of the alkoxy groups remains behind on the surface unreacted. In the course of storage of the products obtained according to the prior art, however, these groups react further in an undesired manner, with the liberation of alcohol. Even during processing in the manufacture of rubber articles, such as during extrusion, the alcohol liberation can lead to pore formation and hence to the impairment of the rubber technology properties of the vulcanizates. Furthermore, during the entire manufacturing process of the rubber article, starting with the mixing process through the further processing of the mixture and up to the vulcanization of the product, alcohol arises and is released into the atmosphere, which no longer complies with current demands regarding workplace hygiene.